Column of blasting agent of controlled density

ABSTRACT

In order to control slurry density in deep holes, varying amounts of aerating or gasifying agent, preferably a generated essentially insoluble gas, are added. Hydrogen peroxide may be decomposed to produce oxygen, using catalyst such as potassium iodide or manganese dioxide, etc. Gas generating material and catalyst may be added variably, e.g. to produce gas in progressively decreasing proportions as slurry is pumped, starting at the bottom and rising in a borehole, to offset the compressive effect of the column weight thus keeping the whole slurry column at density low enough for reliable detonation.

United States Patent [72] Inventors Kay S. Mortensen;

Lex L. Udy, both of Salt Lake City, Utah [2]] Appl. No. 764,195 [22]Filed Oct. 1, 1968 [45] Patented Nov. 2, 1971 [73] AssigneeIntermountain Research and Engineering Company [54] COLUMN OF BLASTINGAGENT OF CONTROLLED DENSITY 9 Claims, 6 Drawing Figs.

[52] U.S. Cl 149/2, 149/41, 149/44, 149/60, 149/61 [51] Int. Cl C06b19/00 [50] Field of Search 149/2, 45,

[56] References Cited UNITED STATES PATENTS 3,288,658 11/1966 Fergusonet al. 149/2 3,288,661 11/1966 Swisstack 149/60 3,369,945 2/1968 Craiget al. 149/44 X 3,382,117 5/1968 Cook 149/44 X 3,390,028 6/1968 Fee etal. 149/44 X 3,390,030 6/1968 Fee et al.. 149/44 X 3,390,032 6/1968Albert 149/44 X 3,447,978 6/1969 Bluhm 149/44 X 3,449,181 6/1969Armantrout et al.. 149/44 X 3,453,158 7/1969 Clay 149/44 3,470,0419/1969 Gehrig 149/2 Primary Examiner-Carl D. Quarforth AssistantExaminer-Stephen .l. Lechert, Jr. Attorney- Edwin M. Thomas COLUMN OFBLASTING AGENT OF CONTROLLED DENSITY BACKGROUND AND PRIOR ART A numberof slurry blasting agents have been proposed and used in recent years inlarge quantities for blasting hard rock and other analogous materials.These commonly are made up of a water solution of strong oxidizer salts,such as ammonium nitrate, or equivalent, which may include sodiumnitrate and other materials. To these are added fuels and sensitizers,usually in solid particulate form and insoluble in the liquid phase,such as carbonaceous materials, finely divided aluminum metal, particlesof self explosives such as TNT and the like. The fuel-sensitizedexplosives, other than those which employ self-explosives, usually tendto become increasingly difficult to detonate when their densitiesincrease. For example, they may become more and more insensitive as theyare compressed to higher than normal densities. A certain amount ofaeration or gas inclusion is usually included in such slurries,purposely or otherwise. See U.S. Pat. Nos. 3,379,587 and particularly3,382,l 17 to Cook, for example.

In columns of slurry placed in shallow boreholes this normally presentaeration, which will be understood herein to cover inclusion of othergases as well as air, is often sufficient to keep the density withinworkable limits. ln deeper columns hydrostatic pressures may compressthe slurry enough to alter its blasting or detonation properties.Densities greater than 1.4 for grams/cc. for some widely used types ofslurry, especially certain aluminum-sensitized slurries containingammonium nitrate and the like in the liquid phase, are often too greatto permit reliable detonation, although this may vary considerably withparticular compositions and with ambient and compositional temperaturesat the time of detonation. For an aluminum sensitized slurry of onetypical composition, for example, it may be quite difficult to detonateat a density of 1.42 g./cc. at normal temperature. The same slurry maybe much less difficult to detonate at a density of 1.3 g./cc. and quiteeasy to detonate at a density of, say, 1.2 g./cc. at the sametemperature. it should be noted that while the density is beingdecreased, the bulk strength of the explosive, which is a function ofits explosive power, as well as its weight, may also be decreased,although bulk strength does not necessarily decrease linearly and maynot decrease at all in some particular compositions. Obviously, it maybe desirable, for this reason, to keep the density of a blasting agentas high as possible, if it can be consistently and reliably detonated,for the purpose of obtaining a maximum blasting effect. On the otherhand, economy may dictate aeration to get greater volume of explosivefrom a given weight of ingredients, even at some sacrifice of bulkstrength. If the slurry density, however, becomes so great due tocompression, deaeration, or any other cause, that the explosive cannotbe detonated at all, or not reliably, the composition, of course, isquite useless.

Various methods have been proposed in the past for incorporating air orother gases in blasting agents of this type. in many slurries, aerationwill be included without addition of any special gas-producingcomponents, i.e. by ordinary mixing operations. Thus, in the Cook U.S.Pat. No. 3,382,117 it is pointed out that a reduction in densitynormally occurs in slurries and that such reductions, up to 25 percentor so, which are due simply to incorporating air into the mixture, notonly can be tolerated but may be very useful under some circumstances.The incorporation of air or other dispersed gas in fine bubbles and welldistributed throughout the slurry is known to add to its sensitivity todetonation. Other references in addition to those mentioned above havesuggested various other means in which aeration may be accomplished. Forexample, Swisstack, in U.S. Pat. No. 3,288,661, suggests use ofa carbondioxide generating agent. Carbon dioxide doubtless can be useful in somecases but it is not particularly suitable for purposes of this inventionbecause its solubility in water varies widely with temperature andpressure.

In large scale mining, as commonly practiced in the U.S., for example inlarge iron ore mines', it is common to drill large boreholes which maybe from 4 to 12 inches in diameter and extending to a depth which mayvary from 20 feet to as much as 50, 70, feet, or up to feet or more.Holes of 3 to 4 inches diameter and great depth are rather commonly usedin some areas and these give particular difficulties in achievingdetonation. In a hole of 20 feet depth or less, the weight of thesuperimposed column of a slurry explosive of density of, say, 1.2 or 1.3g./cc., is not likely to be so great as to cause substantialdesensitization due to excessive compression on the lower part of theblasting charge. However, when holes are deeper, say 50, 60, 70 or up to100 feet or more, as is sometimes practiced, the weight of the column,particularly on the lower portion of the charge, becomes so great thatthe normal aeration or dispersed gas content present may become quiteineffective for its needed sensitization effect. That is, the normalaeration of an uncompressed slurry is insufficient in many cases to keepthe bulk density of a slurry under compression down to a required levelfor good sensitivity and reliable detonation.

Various suggestions have been made in the past for changing thecomposition of a slurry explosive to be placed at different levels in aborehole, an example being Clay et al., U.S. Pat. No. 3,303,738. A morepowerful explosive may be needed at the bottom of a borehole than at thetop. This reference suggests methods or means for changing ingredientsor the proportions of certain ingredients from time to time duringfilling of a singleborehole to incorporate in the slurry variousproperties required at different depths in the hole. Obviously, gascontent or aeration is one property which can be changed as described insaid patent, but generally such changes have been accomplished bychanging proportions of one or more solid or liquid components.

ln many cases, however, it is undesirable to change the composition byaltering proportions, for example, of the major solid or liquidingredients of a blasting slurry. Such a change may introduceundesirable variables in some cases. A good, well balanced explosivecontaining, for example, an aqueous solution of oxidizing salt, such asammonium nitrate with or without sodium nitrate, etc., may be well andproperly sensitized by carefully selected sensitizer such as a smallamount of finely divided aluminum. it may be oxygen balanced with otherfuels, such as carbonaceous materials, sulfur and the like, as is nowwell known in the art. To change proportions of any of these componentsto any substantial degree might be uneconomical of blasting power. Whilethe present invention does not preclude substantial changes in solidand/or liquid ingredients, it may be highly desirable to be able tomerely control the density of the explosive composition, or to keep itconstant or substantially constant at different levels in along'vertical column, without significantly changing its composition sofar as oxidizer and fuel constituents are concerned. That is, the slurrymay be entirely satisfactory and efficient in its combustion and in itsblasting effects, as long as it is not too dense to detonate reliably.To keep its maximum density below a desired maximum level throughout thecolumn may be all that is needed in many cases.

One object of the present invention, therefore, is to make possible thecontrol of density of a slurry, whether variable or standard incomposition, as far as liquid and solid components are concerned, bysimply controlling its gas content or its degree of aeration with finelydispersed insoluble gas bubbles. This may be done by variably adding asuitabie gas to the slurry in any desired manner, preferably by adding agas-supplying agent in controlled and often variable proportions relatedto the column depth. For example, a uniform or substantially uniformdensity in a column of slurry may be maintained throughout the length ofa slurry column in a deep borehole, from bottom to top. The normaluncompressed density of the explosive may be reduced sufficiently bysubstantially greater aeration or gas inclusion at the bottom of thehole to compensate for the compressive force of the weight of thecolumn, such aeration being decreased gradually or stepwise as the holeis filled.

According to the present invention then, a borehole of great depth maybe filled with a standard or even a variably proportioned explosivecomposition whose effective density throughout the depth of the hole canbe kept constant, if desired, or can otherwise be controlled or variedto insure proper sensitivity to detonation at all levels in the column.This is accomplished quite simply according to a preferred aspect of thepresent invention, by progressively varying the amount of an aeration orgas inclusion agent, or a material which will generate such an agent.The progressive variation may be stepwise or by steady and uniformdecrease in gasgenerating material changing as a borehole is filled. Theappropriate material or materials are incorporated into the slurry as itis poured or pumped to the borehole. The proportions of at least onesuch gas-generating material can be substantially directly related, in apreferred case, to the depth at which the borehole is being filled atthe moment.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 shows graphically thedensity-sensitivity properties of certain typical slurry explosives.

FIG. 2 shows graphically the effect on density of adding one gasgenerating agent in various proportions to explosive slurries ofdifferent densities.

FIG. 3 shows a relationship graphically between pressure and density oftypical slurries.

FIG. 4 is a diagrammatic view, in elevation, of an apparatus for mixingand pumping an explosive slurry with variable aeration or gas content.

FIG. 5 shows typical time and feed rate correlations.

FIG. 6 shows diagrammatically a simple system for controlling an initialfeed rate and a controlled decrease rate for a fluid component ofaslurry composition.

DESCRIPTION OF PREFERRED EMBODIMENT The invention will be described withreference to typical and more or less conventional aluminum sensitizedaqueous explosive slurries containing ammonium nitrate and various otheringredients which are commonly employed in such slurries. it will beunderstood, however, that the principles of the invention are notlimited to any particular slurry compositions, being applicable tovarious oxidizer-sensitizer type slurries which display the phenomenonof having their sensitivity decrease as they increase in density, i.e.as they are subjected to increasing pressure or compression.

With many explosives, including fuel sensitized slurries, sensitivity todetonation increases more or less inversely to density. In some casesthe relationship is quite linear. With slurries of this invention, it isoften found desirable, in order to control sensitivity, to decreasedensity of in-place explosives by some aeration or gas inclusion but, asa rule, the density should not be decreased excessively by this means.

Ordinarily, the density of the slurry should not be reduced to as littleas 1.0 g./cc. for incorporation in a borehole where water is present.Otherwise incoming ground waters would tend to lift the slurry out ofits intended position. Floating on water, for example, might move theslurry up a borehole and completely out of place. However, there may besome cases, as where there is no ground water present, where an evenlighter slurry than 1.0 density is desired. It should be noted that someother kinds of desensitization than that due to deaeration or gascompression may occur in some aluminized slurries, e.g. when theirdensity drops lower than about 0.5 to 0,6 gJcc. Excessive densityreduction usually is not desirable because the bulk strength of theblasting agent becomes low and the slurry is likely to become relativelyweak or ineffective in its blasting performance.

There may be additional hydrostatic pressures, above that due toexplosive column height, exerted on the borehole by inflowing water, asin the case of excessive ground water which may exert an upward force onthe bottom of the slurry column resisted by friction between the slurryand the borehole wall. However, the pressure on the slurry at the bottomof the borehole ordinarily may be assumed to be caused by the sum ofweight of the column of slurry plus any head of water or other materialson top of the slurry, including stemming which may be placed on top ofthe column.

It has been found experimentally that the critical diameter of most fuelsensitized slurries, particularly aluminum-sensitized ammonium nitrateslurries, etc., which include some gas or aeration, increases aspressure on the slurry increases, at a rate of about 0.07 to 0.1 inchesof critical diameter in the slurry, per pound per square inch ofpressure increase. That is, a typical slurry of critical diameter d of4inches at zero or atmospheric pressure may have a d, of about 6 inchesat 20 p.s.i.g. Taking a slurry column depth and multiplying it byaverage density of the slurry, plus the depth of any water present ontop of the slurry when the charge is in place, this sum being multipliedby 0.433, will give the pressure in pounds per square inch at the bottomof the hole. This pressure, of course, reduces as one goes up theborehole. From this it will be seen that the critical diameter ofagas-containing or aerated slurry may increase quite rapidly in a deephole. This must be compensated for by density control, particularly whensensitivity approaches a level where detonation failure might beencountered.

The sensitivity to detonation of aluminum-sensitized slurries which arebelow their theoretical or unaerated densities is known to decrease at afairly regular or predeterminable rate as the slurry is subjected toincreased static pressure or compression. According to one aspect of thepresent invention, an agent is added to the slurry which will decomposefairly soon to produce a gas or aerating agent. The aerating gas shouldbe produced in proportion to the need to density control at anyparticular depth, where control of sensitivity is the mainconsideration, as it often is.

Hydrogen peroxide is a preferred gas generator, although other materialsmay be used. When added to the slurry under appropriate conditions andwith a suitable catalyst, it will decompose into water and oxygen soonafter the explosive is in place in the borehole. The oxygen gas sogenerated does not dissolve appreciably in the liquid phase and thuslowers the density of the slurry, offsetting reduction in sensitivitydue to compression or column weight. At the bottom of a deep borehole,considerable aeration to prevent a compression-induced increase indensity may be required. Midway of the borehole, the density decreasedue to superimposed slurry, water, stemmings, etc., may be much less. Ator near the top of the explosive column it may be entirely unnecessaryto have any gasification agent at all since the density may not besubstantially affected by the borehole filling. Preferably, the amountof gas-producing material should be adjusted to meet requirements.

Under some circumstances, it may be desirable and simpler to add thegas-generating substance at a fixed and invariable rate, as the slurryis being mixed and delivered to a borehole. In this case, sufficienthydrogen peroxide or equivalent material is added at first to make surethat the compressed slurry at the bottom of the borehole is sufficientlysensitive for detonation. In this case, slurry higher up the column willbe less dense and consequently more sensitive than necessary fordetonation. It may have less than maximum blasting power too, by reasonof its reduction in bulk density. ln many, and perhaps in most cases,the most powerful part of the charge is needed at the bottom of thehole. A less dense explosive is still adequately powerful higher up andmay be substantially more economical since less weight of explosive isrequired per unit volume.

Hydrogen peroxide is an unstable molecule which rather easily decomposesto give off an extra oxygen atom. By use of a catalyst, or establishmentof other degeneration conditions, such as the aging of a peroxidesolution or the use of materials which release the peroxide gradually,one can cause the peroxide to decompose at reasonably controllablerates. For purposes of the present invention a very desirable catalystis found to be potassium iodide (Kl). Manganese dioxide in proportionsof 0.05 to 1 percent, preferably around 0.2 percent of weight of totalslurry has also been used, as well as ferrous sulfate and mixtures offerrous sulfate, managanous sulfate and an organic compound such as HMT(hexamethylene tetramine). Ferric nitrate, in proportions of up to 0.2percent is effective, although it has showed sometimes a tendency tofail in effectiveness, e.g. after extended storage in the premix whichcontained powdered aluminum and other ingredients discussed more fullybelow. The use of potassium iodide or manganese dioxide, or acombination of manganese dioxide in the premix with ferric nitrate addedto the oxidizer solution, therefore, is presently preferred. However,any catalyst which will cause reasonably rapid decomposition of hydrogenperoxide can be used. It is found that a combination offerric nitratewith manganese dioxide is a better catalyst than either alone. Theferric nitrate can be added in the form ofa 50/50 aqueous solution (byweight) to the solution of oxidizer. Obviously, the decomposition rateof the hydrogen peroxide should be such that the oxygen gas bubblesreleased are very tiny. Being produced and widely distributed in aviscous medium, which explosive slurries usually are or contain, thesmall gaseous bubbles once released are not likely to coalesce or torise to the top to any significant degree. This means that finelydistributed aeration or gasification remains in the slurry, even atgreat slurry column depths and under considerable superimposedpressures. The decomposition preferably should be sufficiently rapidthat the slurry is aerated almost or approximately as fast as it fillsthe borehole, but some delay is permissible.

Hence, with a given explosive slurry composition, such as one comprisedmainly of an aqueous solution of ammonium nitrate, sodium nitrate, etc.,the insoluble sensitizer and/or fuel particles, which are suspended inthe solution to form the slurry, are added in any convenient way andstirred in to make a stable, homogeneous suspension. Particles such asfinely divided gilsonite, sulfur, aluminum powder, etc., are commonlyused. Oxidizer particles, such as ammonium or sodium nitrate, may alsobe suspended in the solution without dissolution because the solution issaturated.

According to one system which is quite satisfactory, potassium iodide isfirst placed directly in the solution in small proportions such as 0.05to 0.5 percent by weight of total slurry, preferably about 0.2 percent.The hydrogen peroxide is metered into the mixing zone or funnel as theother ingredients, such as the solid particles mentioned, are mixedtherein. A small amount, i.e. a fraction of 1 percent of total slurry byweight, is enough hydrogen peroxide to reduce a slurry density from 1.4to 1.0 or less. The proportions of hydrogen peroxide used at thebeginning of a borehole filling operation are high enough to giveconsiderable aeration to that part of the slurry going at the bottom ofa borehole. By appropriate control the amount of hydrogen peroxidemetered as the mixing proceeds for filling a given borehole, is reducedgradually to zero by the time the borehole is filled. In someoperations, the potassium iodide or other catalyst and the hydrogenperoxide additive may be mixed concurrently into the mixing funnel alongwith the fuel and other solid particles which are stirred into thesolution and suspended therein.

According to a preferred embodiment, hydrogen peroxide is placed in atank in a pump truck which contains a slurry mixer, preferably a mixingfunnel. This hydrogen peroxide is metered into the slurry at the mixingfunnel at a controlled and preferably decreasing rate for each boreholebatch, as indicated above. The pump used for dispensing the peroxidepreferably is a peristaltic type, operating by applying a squeezing,pumping action applied to a flexible tube. The rate of pumping thehydrogen peroxide ingredient and, hence, the flow rate thereof, iscontrolled by a zero-Max" speed controller. The latter device, of knowncommercial type, can be set at any pumping speed from zero to its ratedmaximum simply by moving a control lever.

According to a preferred procedure, the speed control lever is preset tothe maximum rate needed for a borehole batch and is automatically movedtoward the zero position over the time required to fill the borehole bya second Zero-Max" mechanism. This second unit may be attached to thecontrol handle of the first and so arranged that it turns the control ofthe first unit back to zero in a predetermined time. By knowing thepumping rate and dimensions of borehole to be filled, this time can bepreset accurately.

In practical use, the density of a slurry ordinarily should not exceed avalue of about 1.3 to L4 g./cc., referring now to aluminum sensitizedslurries, without being desensitized and failing to shoot on detonation.On the other hand, the slurry density as aerated should not be allowedordinarily to go below 1.0 g./cc. Otherwise, it will float on any waterand be lifted out of the borehole, where ground water is present inquantity, as is commonly the case. A simple calculation shows themaximum amount of the gasifying or levitating agent, that is, ofhydrogen peroxide, which is needed at the toe or bottom of the column.None is needed at the top in most cases. The maximum rate is thenadjusted as pumping of the slurry proceeds. According to the presentinvention the reduction in pumping rate is accomplished automatically.The operator needs to set only the desired initial rate and the totalperoxide flow time. The controls then start feeding peroxide as boreholefilling commences and the feed rate declines to zero by the time thehole is fully charged.

Referring now to the drawings, FIG. I shows graphically the effect onsensitivity of variations in density for three different slurries, A, B,and C. FIG. 2 shows the percent of concentrated peroxide which isnecessary to obtain different specified densities for slurry atdifferent pressures. A slurry of density 1.1 g./cc. at zero pressurerequires about 0.8 percent of hydrogen peroxide (35 percent strength) tomaintain the same density under 38 p.s.i. A similar slurry of density1.2 at near zero pressure requires 0.6 percent of H 0 to maintain thesame density under pressure of47 p.s.i., etc. A simple calculation basedon slurry column and desired slurry density (plus depth of water on topof slurry in water filled holes) will show the hydrostatic pressure atthe bottom of the borehole. Hence, bottom pressure is calculated for thedesired density, and the peroxide is metered into the slurry as fillingis started at the indicated rate. The second control is adjusted togradually cut down the peroxide feed rate to zero by the time theborehole is filled. This time can be calculated by knowing the slurryfeed rate and the total charge to be placed in a borehole of specifieddepth, filled with the selected slurry of desired density. Ordinarily,concentrated 35 percent strength peroxide is used and the charts (FIGS.2 and 3) are based on that concentration. This peroxide must be usedwith care because it can be harmful to the human skin.

As a concrete example of a filling operation, a 74 foot borehole to befilled with slurry of density of 1.25 g./cc., FIG. 2, when the normalunaerated density of slurry is about l.4, will require 0.36 percent byweight of 35 percent strength peroxide, based on the total composition.Slurry of 1.25 g./cc. density in a column 74 feet high is under pressureof 40 p.s.i.g. Knowing the percentage of peroxide required, it can bereadily calculated the amount of hydrogen peroxide equivalent to thenecessary flow rate for a slurry pumping rate of, say, 300 lbs. perminute, as an example. Pumping rates of to 300 lbs. per minute or moreare commonly in use in pump truck mixer-type apparatus of the generaltype mentioned above.

If the peroxide is diluted half-and-half with water, as may be desirablefrom the standpoint of safety, the peroxide flow rate, of course, shouldbe doubled to obtain the same quantity of gas on decomposition.

A reading of FIG. 2 together with knowing the desired density of theunaerated slurry and the borehole depth, will permit determination ofthe peroxide flow rate which is necessary to aerate the slurry at thebottom or toe of the borehole. The next step, obviously, is to calibratethe apparatus to give the required peroxide flow rate.

In a specific example, a mixer-pump truck using straight 35 percentconcentrated peroxide, the first step is to set the peroxide pump at asuitable rate and set the second or rate reduction control to zero sothat the peroxide pump will run at a constant speed. A calibration isthen made in volume, determining the cubic centimeters of peroxide perminute which will be pumped at the set rate, using water, for example,as the test liquid. In this way appropriate settings of the peroxidepump are determined. The volume of fluid pumped is approximately linearwith respect to the setting of the pump rate or volume control for theapparatus shown.

Typical compositions aerated in the manner described comprise thefollowing ingredients:

30 to 50 percent by weight of ammonium nitrate 10 to 40 percent ofsodium nitrate 12 to 20 percent of water to percent of sulfur l to 10percent of carbonaceous fuel (gilsonite or bituminous coal) 0 to 10percent of aluminum, preferably 0.1 to 8 percent, a small part of whichpreferably is fine flaked grade, typified by paint grade aluminum 0.2 to2 percent of thickener, such as quar gum, preferably with a small amountof Borax or other cross-linking agent.

A small amount of an inhibitor to prevent premature aluminum-waterreaction is desirable when fine reactive aluminum is used, as set forthmore fully in US. Pat. No. 3,l l3,059.

By use of suitable proportions of sulfur, also, the ammonium nitratecontent may be reduced and proportions of sodium nitrate increased toexceed the proportions of ammonium nitrate.

Ingredients are adjusted typically to bring oxygen limits within :IOpercent and, if aluminum is not included, special combinations of sulfurand carbonaceous fuel, etc., are employed, as is known in the art. Useof about 0.1 to 0.2 percent of a 50 percent solution of potassiumiodide, with appropriate quantities of hydrogen peroxide, as shown inFIG. 2 (up to about l percent) is recommended. The alternative catalystsalready described may be substituted or used as a part of the catalystin suitable proportions and in the manner mentioned above.

For convenience of the operator, a table may be made up listing thesetting of the controls vs. borehole depth for a given composition.Knowing the pumping rate and the original or desired density of theslurry, it is easy to calculate the amount of peroxide or other aerationagent that should be added.

FIG. 4 shows a typical apparatus in general form for mixing slurry andpumping it into a borehole 10. The apparatus comprises a tank 11 for aconcentrated solution of ammonium nitrate, which may include some sodiumnitrate and a small amount of thickener in water. Line 12, having a cutoff valve 13, feeds the solution to mixing vessel 14 having a suitablestirrer or mixing device M. Bins 16A, 16B and 16C supply particulateingredients, such as aluminum, fuels, supplemental oxidizer, etc. Thelatter are fed by augers 17 or equivalent at various rates, depending onthe composition of the slurry. A small tank PT carries a supply ofgas-generating material, preferably hydrogen peroxide. Another tank PIfor solution of catalyst potassium iodide or the like, may be connectedto supply catalyst at a controlled rate. Only a small amount of catalystis needed. The latter flows through a line L, provided with a cutoffvalve to metering pump means described further below. A slurry pump SP,driven by a suitable power source, pumps the finished slurry through ahose or other conduit into borehole 10. A valve V, may be provided tohold slurry in the mixer vessel 14 until pumping is started.

The time of flow of peroxide or other aerating agent is governed by theamount of slurry to be pumped into the borehole. Knowing the pumpingrate, the time is easily calculated. Apparatus shown in FIG. 6 includespump 18 driven by a motor 20 and involves a control 21 for pump motor20. An arm 23 sets control 21 and changes its setting under control of arotating shaft 25 driven by motor 26. The latter has screw threads 31which may be counted to determine the number of rotations of thisparticular element required to accomplish a given result. Control 41 formotor 26 controls the rate at which the threaded shaft 25 is turned andtherefore determines the setting of its control 21. These relationshipsdetermine the elapsed time required for the pump rate of the peroxideadditive to go from maximum to zero.

The time required to reduce the feed rate of pump 18 to zero from itsmaximum starting position, as each borehole is filled, may be measuredexperimentally or calculated from the number of turns or threads on theshaft.

Once these calibrations are made, the operation of the device is quitesimplev The borehole depth determines the maximum or initial rate ofperoxide flow and the pumping time required to fill the borehole givesthe total peroxide flow time. Of course, if desired, the peroxide may becut off before the full charge is inserted since the upper part of thecharge may not be much affected by hydrostatic compression. Assumingthat 500 lbs. of slurry are to be treated with peroxide all the way andplaced in a 40 ft. depth borehole, the chart may be used as follows. The40 ft. borehole depth is used to calculate hydrostatic pressure and,hence, to determine its effect on slurry, per FIG. 2. 500 lbs. of slurrypumped at 300 lbs/min. requires seconds of pumping time. FIG. 5, ofcourse, shows the slope S or rate of decrease to go from maximum feedrate to zero in 100 seconds.

Determining the initial flow rate from FIG. 2, the electrical switchesfor motors 20 and 26 and peroxide valve 15 are turned on. The apparatusruns automatically while the pump delivers the peroxide. When a seriesof holes of equal depth are to be filled, and the slurry is not changedin composition, it is a simple matter to reset the control 21, 23 backto the starting position with each hole. If the slurry pumping time ischanged, or the hole depth from hole to hole. the procedure must bemodified by changing control 41 to compensate for this, Likewise, for achange in composition it is necessary to change the setting of one orboth controls 21, 41.

As noted above, hydrogen peroxide is a strong oxidant and can causefires when spilled on or in any combustible materials. It is hazardousto the body and can cause blistering of skin and severe irritation.I-Iuman eyes are particularly sensitive to it and care should be takenin handling the material. Peroxide solution usually is dyed adistinctive color so that it will not be mistaken for a less energeticor less harmful fluid.

FIG. 5 shows graphically how the apparatus of FIG. 6 may be used tocontrol the feed rate of the gassing agent. The initial feed rate may beadjusted at any desired level such as indicated by the points Q or R. Ifnot changed by the second control 41, the feed would continue at auniform rate, i.e. on a horizontal line Q or R By setting the zero-maxcontroller 41, the rate of rotation of screw 25, FIG. 6, is adjusted todetermine the slope of the desired control rate Q or R etc.Determination of (a) initial feed rate Q or R, plus (b) the requiredpumping time, e.g. 100 seconds, or seconds, or some other time, to filla borehole thus is used to set controls 21 and 41. These will controlpump 18 to incorporate the right amount of gassing agent at any depth inthe borehole. The catalyst from tank PI may also be metered, if desired,by this same pump or by another. Usually, it is not necessary to changecatalyst feed rate but it may be desirable under some circumstances.

While substances other than hydrogen peroxide may be substituted, H O ispreferred for the purpose of the present invention. It will be obvious,however, to those skilled in the art that other materials producingother gases, e.g., carbon dioxide, etc., could be used and the sameprinciples apply.

We claim:

1. A tall aerated blasting charge in the form ofa compressible column ofexplosive composition having tendency to lose sensitivity underincreasing density conditions, which comprises an elongated detonablemass of thickened blasting slurry having a continuous or substantiallycontinuous liquid phase and including an oxidizer salt in solution insaid liquid phase, sensitizer-fuel material distributed throughout saidsolution, and an aerating gas finely dispersed and distributed in atleast the lower part of said column in proportions at least sufficientto maintain detonability throughout the entire column of explosive byoffsetting loss of sensitivity in the slurry due to the increase ofdensity in the composition resulting from compression of said column.

2. A blasting charge according to claim 1 wherein proportions of saidaerating gas decrease from the bottom towards the top ofthe column.

3. A blasting charge according to claim 1 which contains decomposiblehydrogen peroxide as an aerating agent.

4. A charge according to claim 1 which contains hydrogen peroxide as anaerating agent and a small proportion of hydrogen peroxide-decomposingcatalyst, said catalyst being uniformly distributed and said hydrogenperoxide being nonuniformly distributed throughout said charge.

5. A blasting charge according to claim 1 which contains sufficientaerating gas to keep its bulk density in all parts of the column belowabout 1.4 grams per cubic centimeter.

6. A blasting charge according to claim 1 which contains a substantiallyuniformly decreasing proportion of aerating gas from the bottom of theexplosive column to the top.

7. A charge according to claim 1 wherein the gas is oxygen.

8. A charge according to claim 1 which includes a gasgeneratingsubstance unequally distributed with a high concentration in the lowerpart of the charge and a lesser concentration in the upper part.

9. A charge according to claim 1 which comprises a substantiallyuniformly distributed small concentration of gas producing agentsufficient to insure detonation at the bottom of a deep column of slurryand more than adequate for insured detonation at higher levels.

t i i

2. A blasting charge according to claim 1 wherein proportions of saidaerating gas decrease from the bottom towards the top of the column. 3.A blasting charge according to claim 1 which contains decomposiblehydrogen peroxide as an aerating agent.
 4. A charge according to claim 1which contains hydrogen peroxide as an aerating agent and a smallproportion of hydrogen peroxide-decomposing catalyst, said catalystbeing uniformly distributed and said hydrogen peroxide beingnonuniformly distributed throughout said charge.
 5. A blasting chargeaccording to claim 1 which contains sufficient aerating gas to keep itsbulk density in all parts of the column below about 1.4 grams per cubiccentimeter.
 6. A blasting charge according to claim 1 which contains asubstantially uniformly decreasing proportion of aerating gas from thebottom of the explosive column to the top.
 7. A charge according toclaim 1 wherein the gas is oxygen.
 8. A charge according to claim 1which includes a gas-generating substance unequally distributed with ahigh concentration in the lower part of the charge and a lesserconcentration in the upper part.
 9. A charge according to claim 1 whichcomprises a substantially uniformly distributed small concentration ofgas-producing agent sufficient to insure detonation at the bottom of adeep column of slurry and more than adequate for insured detonation athigher levels.